Note: the class constraint that a is an instance of LookupEntryClass
is not defined here, for good reasons (which I forget right now, but
something to do with the method dictionary being superfluous on
an algebraic data type).

Find a node in a lookup map list, and returns the
corresponding value from a supplied list. The appropriate ordering
of the list is not specified here, but an appropriately ordered list
may be obtained by mapApplyToAll.